§ 1.1 Field of the Invention
The present invention concerns locating an object of interest, such as a mobile or stationary object. More specifically, the present invention concerns locating an object of interest radiating signals in an environment which induces multiple ray paths of the radiated signals.
§ 1.2 Related Art
§ 1.2.1 The Need to Locate Objects of Interest
There are a wide range of applications that could benefit from improved object of interest location techniques. One general application involves locating a scene at which emergency services (e.g., medical, fire, accident, crime, etc.) are needed. In many of these emergency applications, the person attempting to obtain assistance (e.g., with a 911 call) may be in a state of panic, or simply may not know their present location. Regarding medical emergencies specifically, the person attempting to obtain help may be partially incapacitated and unable to provide their present location.
Other potential uses for improved object of interest location techniques may include tracking persons (e.g., prisoners, workers), vehicles (e.g., delivery vehicles, service vehicles, pubic transportation vehicles, private taxi/limousine services, stolen vehicles), cargo, etc.
Object of interest locating techniques could also be useful for aiding a person who is lost, e.g., driving in an unfamiliar city. Such mobile location techniques and improvements could be coupled to mobile communication devices and/or navigation systems providing additional information.
Improved object of interest locating techniques could also be beneficial in a military environment. For example, it would be advantageous if each member of a friendly unit or each vehicle in the unit could be located and/or tracked, e.g., within a hostile city environment, even where obstructions might render other currently used location techniques ineffective. It would also be advantageous if an enemy or hostile unit, individual, or vehicle could be located and/or tracked, e.g., within a city environment, by e.g., their cell phone communications and/or a signature emitted by any of their electronic equipment, even when obstructions might render other currently used location techniques ineffective or too imprecise. In such an application, location techniques that are passive would have any advantage over other techniques, e.g., active radars, which could alert the enemy that they are being monitored.
Note that in many of the above applications, the object of interest cannot (e.g., in the case of a disoriented accident victim), or will not (e.g., in the case of a military enemy target) provide location information.
For many applications of interest it is only a two dimensional (2-D) location of the object on the earth's surface is desired. In other applications it is also desirable to know the elevation of the object above the earth's surface.
In many of the abovementioned applications, response time is a critical factor. Therefore, any improvement over present techniques or any additional location confirmation provided by new locating techniques would be beneficial.
§ 1.2.2 Known Techniques For Determining an Object of Interest's Location and Their Perceived Drawbacks
Known techniques for determining an object of interest's location and their perceived drawbacks are introduced in §§ 1.2.2.2 through 1.2.2.4 below. First, however, the concept of multipath rays/signals is introduced in § 1.2.2.1.
§ 1.2.2.1 Multipath Rays and Their Sources
From a source, e.g., a transmitter, an electromagnetic wave may propagate radially, defining a number of rays. The wave's subsequent paths through space may be determined by the laws of ray optics, including geometrical optics and the uniform theory of diffraction. Geometrical optics refers to the process by which high frequency electromagnetic waves are represented in terms of rays consisting of straight-line segments between the source and points of reflection. Reflection occurs when a ray encounters a surface (e.g., a side of a building) and leaves the surface in a single direction, in which the angle of incidence equals the angle of reflection. Diffraction refers to a wave process by which a geometrical ray is scattered into many directions lying on the surface of a cone by the sharp edges or other abrupt changes in the properties of a surface, (e.g., the corner of a building).
Multipath refers to the condition of having electromagnetic waves arriving at a point in space along many different ray paths. Multipath rays arriving at one observation point (referred to simply as “the observer”) may include a mixture of rays sourced from a plurality of transmitters situated at different locations. The multipath rays may include a subset of rays transmitted from a single source. Since rays emanating from a single source at a given time may have taken different paths, the rays may arrive at the observer at different points in time, with different amplitudes, and/or with different angles of arrival.
The multipath rays emanating from a source to an observer may reach the observer directly, or after one or more reflections, and/or diffractions. The rays may be classified in four main categories: (i) direct propagation; (ii) rays that experience reflection at generally vertical surfaces (e.g., sides of buildings); (iii) rays that experience diffraction at generally vertical surfaces (e.g., vertical corners of buildings); and (iv) rays that experience a combination of reflection and diffraction at generally vertical surfaces (e.g., the sides and corners of buildings). Note, however, that rays may also undergo reflection and/or diffraction from generally horizontal surfaces (e.g., reflection from the ground and/or diffraction over the tops of buildings). In such cases, the general 2-D ray classifications can be sub-classified as (i) direct with horizontal reflection and/or diffraction; (ii) reflection only with horizontal reflection and/or diffraction; (iii) diffraction only with horizontal reflection and/or diffraction; and reflection and diffraction with horizontal reflection and/or diffraction. Although such rays can be projected onto a 2-D horizontal plane, in such cases, a signal time and/or amplitude should further account for delays and/or attenuation due to reflections and/or diffractions from generally horizontal surfaces.
Thus, direct propagation rays are those that have not encountered obstructions in their paths from source to observer. In such cases, a line of sight is said to exist between the source and observer. This category may also include the sub-category of rays that undergo reflection from the ground and/or pass over buildings by the process of diffraction at the horizontal building edges.
Reflected rays include those that have undergone one or more reflections at generally vertical surfaces (e.g., sides of buildings) but no diffractions at the vertical corners (e.g., of buildings) along their paths from source to observer. This category may also include the sub-category of rays undergo reflection from the ground and/or go over some buildings via diffraction at the horizontal building edges.
Diffracted rays include those that undergo diffraction at generally vertical surfaces (e.g., corners of buildings), but and no reflections (e.g., at the vertical sides of buildings) along its path from source to observer. This category may also include the sub-category of rays that undergo reflection from the ground and/or go over some buildings via diffraction at the horizontal building edges.
A ray that is classified as a combination of reflection and diffraction is one that has undergone at least one reflection at a generally vertical surface (e.g., sides of buildings) and at least one diffraction at a generally vertical corner (e.g., of buildings) along its path from source to receiver. Again, this category may also include the sub-category of rays that undergo reflection from the ground and/or go over some buildings via diffraction at the horizontal building edges.
§ 1.2.2.2 Triangulation and its Perceived Drawbacks
Triangulation measures at least one of the relative time delay, angle of arrival, and amplitude of a signal received at three or more observation points (e.g., base stations) surrounding an object of interest. In the absence of multipath, signals at an observation point arrive directly from an object of interest source with time delay proportional to the distance R, and amplitude inversely proportional to the a power of R. Under these circumstances, the object of interest can be located using any one of the signal properties.
Unfortunately, however, in some environments (e.g., an urban environment), direct propagation paths are not always available between the source and the three observation points. For example, in an urban environment with multipath due to scattering by buildings, it is not uncommon for signals to arrive with angle discrepancy on the order of +/−10°, which corresponds to +/−160 m at a mobile distance of R=1 km from the observation points. Multipath may also result in signals arriving with delays of more than 1 μs, which corresponds to a distance of 300 m. Buildings may also cause shadow fading that may result in variations of the signal of more than +/−6 dB. For signals that depend on distance as 1/R4, fading of +/−6 dB is equivalent to a distance variation of 30%, or 300 m at an R=1 km from the observation points. Thus in the presence of buildings or other obstacles to direct ray propagation, the accuracy of the triangulation methods may be severely degraded.
Another drawback to the triangulation method is the need for three receivers at three locations with synchronization and information exchanges between the three locations.
§ 1.2.2.3 GPS and its Perceived Drawbacks
A second approach for locating an object of interest uses a Global Positioning Satellite (GPS) receiver in the object of interest. The GPS receiver uses the time difference of three or more satellite signals to triangulate its location. The accuracy in determining the elevation of the receiver is largely dependent on satellites that are positioned overhead. However, the accuracy of the latitude and longitude is dependent on receiving the signals from satellites at low elevation angles. The signals from these satellites are subject to the same multipath errors that exist when the object of interest is a transmitter rather than a receiver. As a result, the GPS approach can be susceptible to errors in the determined horizontal position of the object of interest similar to those for the triangulation method described above.
Moreover, with some GPS receivers, it is not unusual to be unable to obtain a position fix at numerous locations within a city environment. At many locations within the city, the receiver's antenna may not have a line of sight path to a minimum required satellite set for any position fix. The GPS receiver may have been implemented to filter out received multipath signals to improve accuracy under good observation conditions. Unfortunately, this limits reception in a city environment.
These drawbacks of GPS are in addition to the need for hardware and/or software modifications to the object of interest (e.g., incorporation of an embedded GPS receiver).
Finally, from the viewpoint of an observer, the GPS receiver method is only useful in applications in which the object of interest has a GPS receiver, and perhaps voluntarily shares its location information. In addition, with the GPS method, the sharing of location information between object of interest and observer involves additional communications hardware, the establishment of a communications link, and additional signaling.
§ 1.2.2.4 “Radio Camera” and its Perceived Drawbacks
A third approach for locating an object of interest attempts to use the complete information of the received signal to identify location. Known as location signature or “Radio Camera,” the approach requires knowledge of one or more characteristics of the received signal from each of a number of possible candidates object locations to each of a number of observation points (e.g., base stations). Measuring the characteristics obtained at many observation points, a computer program attempts to find the best match to a database of previously recorded signatures. Although the question of uniqueness of the signature is potentially a theoretical limit on the accuracy of the method, it is probably less significant than other mundane sources of error. Major drawbacks of the “radio camera” method are associated with the database of signatures, which is obtained by extensive measurements or predictions. If the database is constructed for very fine grid of locations, it may require a prohibitive number of measurements and be very difficult to manage. For example, if the signature at 10 base stations is used for a 5 m grid then there may be 0.4 million entries needed for a 1 km2 area.
Using a coarser grid can cause problems since the signatures (such as spread of the received signal in angle of arrival, the spread over time, and received power) can vary by a large amount for displacements of the object by distance of the order of 10 m. Thus a database obtained while driving down the middle of the street may result in errors for pedestrians on the sidewalk next to the buildings. Moreover, trucks, and other moving objects can block or scatter signals leading to changes in the signal that cannot be accounted for.